1. Field of the Invention
This invention relates to reflective articles comprising a reflective metal coating deposited on a glass substrate, and it extends to methods of manufacturing such articles.
The present invention results from research which was conducted with the principal aim of conserving the optical properties of glass bearing reflective coatings containing silver by protecting such coatings against atmospheric corrosion.
A reflective metal coating may be deposited pattern-wise to form a decorative article, but the invention has particular reference to glass substrates bearing a continuous reflective coating. The reflective coating may be so thin that it is transparent. Glass panes bearing transparent reflective coatings are useful inter alia as solar screening panels or as low-emissivity (in respect of infra-red radiation) panels. Alternatively, the coating may be fully reflective, thus forming a mirrorcoating. Such a mirror may be plane, or it may be curved.
2. Description of the Related Art
Reflective metal coatings, e.g. of silver, are apt to be attacked by atmospheric pollution with the result that the silver layer becomes tarnished so that the required optical properties of that layer are lost. It is accordingly known to apply protective layers to such a silver layer, the nature of the protective layer being determined by the required properties of the coated substrate and by cost.
For example, transparent silver layers such as may be used in solar screening coatings may be protected against corrosion by overcoating them with one or more transparent metal oxide layers. Such silver layers are often formed by a vacuum deposition technique, and the protective layer(s) is or are also formed by vacuum deposition, often in the same apparatus, to avoid risk of marring the silver layer. Such protective layers are expensive to form.
Front-silvered mirrors may be protected in the same way.
Back-silvered mirrors may be protected by one or more opaque layers, since the optical properties of the rear face of a mirror are largely irrelevant, and that face is anyway usually hidden from view in some form of mirror mounting.
According to classical methods, mirrors are manufactured by sensitizing a glass sheet, applying a silvering solution to form a silver reflective layer, overcoating that silver layer with a protective layer of copper, and then painting the copper layer to produce the finished mirror.
The purpose of the copper layer is to retard tarnishing of the silver layer, and the copper layer is itself protected from abrasion and corrosion by the layer of paint.
Of the various paint formulations which could be used for protecting a mirror, those which afford the best protection against corrosion of the copper layer contain lead pigments. Unfortunately lead pigments are toxic and their use is being increasingly discouraged for reasons of environmental health.